Polyimide is the first choice among high performance polymer materials because of its excellent thermostability and desirable mechanical, electrical and chemical properties. Moreover, as requirements on semiconductor performance have become increasingly rigorous, practical limitations and deficiencies of conventional inorganic materials have grown more pronounced. These limitations and deficiencies can be offset in certain aspects by the properties of polyimide. Thus, the development of aromatic polyimide by Du Pont Corporation has attracted extensive attention, resulting in development of a variety of polyimides with multiple uses.
In the semiconductor industry, polyimides have been widely used in passive film, stress buffer film, a-particle masking film, dry etching mask, micro-electromechanical systems, interlayer insulating film, etc.; other new applications are continually being developed. Protective coating for integrated circuit devices is a predominant application, since polyimide materials have passed reliability testing for integrated circuit devices. However, polyimide is not only applied in the integrated circuit industry, but is also a key material in electronic packaging, enameled wire, printed circuit boards, sensing elements, separation film and construction materials.
Typically, polyimide is synthesized by two-stage polymerization condensation. In the first stage, a diamine monomer is dissolved in a polar, aprotic solvent such as N-methylpyrrolidone (NMP), dimethyl acetamide (DMAC), dimethyl formamide (DMF) or dimethyl sulfoxide (DMSO), and then an equimolar dianhydride monomer is added to the solution, followed by condensation at low temperature or room temperature, to form polyimide precursor, i.e., polyamic acid (PAA).
In the second stage, dehydration-condensation and cyclization reactions are carried out by thermal imidization or chemical imidization to convert polyamic acid into polyimide. To obtain a polyimide polymer with excellent electrical and physical properties typically requires heating for several hours at a high temperature of 300 to 400° C. in thermal imidization to form the highly imidized polymer. However, due to temperature restrictions inherent to some semiconductor processes, greater attention is gradually being paid to materials that can induce polyamic acid be imidized at a low temperature.
In some applications, addition of a base will promote crosslinking of monomers to cure them into a polymer. However, direct addition of the base to a formulation composition would give rise to disadvantages such as reduced storage stability. Therefore, a technique has been developed to delay the effect of the base by providing a base generator in which a base is protected by a protecting group and will be generated after the base generator is exposed to heating or irradiation of light.
Amines are commonly added as the base to catalyze low-temperature imidization. However such amine compounds are likely to catalyze imidization at room temperature. Mitsuru Ueda et al. developed a series of alkylamine thermal base generators (TBGs), as disclosed in Chemistry Letters, Vol. 34, p. 1372-1373 (2005); JP 2007056196A and Journal of Photopolymer Science and Technology, Vol. 21, No. 1, p. 125-130 (2008). Although the alkylamine thermal base generators can be used to catalyze imidization, the polyimide polymer film obtained therefrom suffers inferior thermal and mechanical properties.
The present invention represents the culmination of research and development on the problems mentioned above. The inventors of the present invention found a novel base generator which can be used in imidization for the preparation of polyimide and is effective in lowering the cyclization temperature of polyimide and improving the thermal and mechanical properties of the polyimide polymer, so as to meet demands in the industry.